The Birth of Be Star Disks II. A High-Resolution Spectroscopic Campaign and TESS Observations of an Outburst of the Classical Be star λ Pavonis

Technical Summary: The Birth of Be Star Disks II. A High-Resolution Spectroscopic Campaign and TESS Observations of an Outburst of the Classical Be star λ Pavonis

Problem and Context
Classical Be stars are rapidly rotating B-type stars characterized by circumstellar Keplerian decretion disks that produce Balmer-line emission. While the Viscous Decretion Disk (VDD) framework successfully describes disk evolution, the specific mechanisms triggering disk formation (outbursts) and dissipation remain poorly understood. A primary hypothesis involves non-radial pulsations (NRPs) transferring angular momentum to the stellar surface to drive mass ejection. Previous work by J. Labadie-Bartz et al. (2025) (Paper I) identified 30 disk outbursts in 13 stars, including λ Pavonis (HR 7074), a B2IIIe star in the Scorpio-Centaurus association. Paper I focused on Hα behavior and circularization but lacked a multi-line analysis and a detailed connection between photometric pulsations and spectroscopic line-profile variations (LPVs) during a pristine diskless-to-disk transition. This paper addresses the need to characterize the full temporal evolution of a disk outburst—from a diskless state through growth, circularization, and decay—while simultaneously analyzing the star's pulsational behavior to distinguish between photospheric and circumstellar signals.

Methodology
The study utilizes a contemporaneous dataset comprising 698 high-resolution spectra and TESS photometry from 2023.

• Spectroscopy: Data were obtained using the CHIRON spectrograph (R ~ 80,000) on the CTIO 1.5-m telescope and the NRES spectrograph (R ~ 53,000) on the Las Cumbres Observatory 1.0-m telescopes. The campaign covered the period from June to September 2023, capturing the star in a diskless state, the onset of an outburst, and the subsequent decay.
• Photometry: Five TESS sectors were analyzed: Sector 13 (2019), Sectors 66–67 (2023, capturing the outburst onset), and Sectors 93–94 (2025).
• Analysis Techniques:
  • Equivalent Width (EW) and V/R Ratios: Time-resolved EW and violet-to-red (V/R) ratios were measured for seven optical lines: Hα, Hβ, He I λ4713, λ4921, λ5876, λ6678, and Si III λ5739.
  • Modeling: Disk growth and decay were modeled using an exponential function fitted via Markov Chain Monte Carlo (MCMC) sampling (emcee). V/R variations were modeled using a sinusoid modulated by an exponential decay.
  • Frequency Analysis: Fourier analysis (Period04) was performed on TESS light curves and spectroscopic line profiles to identify periodicities. Pre-whitening techniques were used to isolate dominant signals.
  • LPV Characterization: High-cadence intra-night spectroscopic sequences (nine dense nights) were analyzed to characterize fast-moving substructures in line profiles.

Key Results

1. Disk Outburst Evolution: The star transitioned from a pristine diskless state to a full disk over a short timescale.

   • Build-up: Emission in all lines (H I and He I) began concurrently around HJD 2460152 (July 26, 2023). The disk built up within approximately 4 days.
   • Circularization: The disk circularized (azimuthal homogenization) in about 12 days, evidenced by the stabilization of V/R ratios with a period of ~1.25 days.
   • Decay: Decay occurred hierarchically. Higher excitation He I lines (λ4713, λ4921) dissipated first (2.4–3.9 days), followed by lower excitation He I lines (3.4–5.7 days), and finally H I lines, with Hα taking the longest (~8.5 days) to return to continuum levels. This supports an "inside-out" dissipation model where inner, hotter regions clear faster than outer, cooler regions.

2. Photometric and Pulsational Signals:

   • Dominant Signal: The strongest photometric signal is a periodicity at 0.163 cycles d⁻¹ (6.13 days), present in all TESS sectors and stable over a six-year baseline.
   • Frequency Relation: This 0.163 d⁻¹ signal corresponds to the difference between two higher-frequency signals detected in TESS: ν₂ = 1.644 d⁻¹ and ν₃ = 1.485 d⁻¹.
   • Spectroscopic Correlation: The 0.163 d⁻¹ difference frequency is clearly visible in spectroscopic LPVs as a prograde (blue-to-red) migration of absorption features. However, the two "parent" frequencies (1.644 and 1.485 d⁻¹) are not detected in the spectral lines before, during, or after the outburst, likely due to interference from fast, non-photometric LPVs and the dominance of the difference frequency signal.
   • Fast LPVs: High-cadence observations revealed fast-moving, highly structured substructures in line profiles (timescales of hours) that appear as both prograde and retrograde features. These features are visible in photospheric lines (Si III) and circumstellar lines, persisting through diskless, outburst, and decay phases. They show no corresponding photometric signal, suggesting they are high-order pulsation modes (e.g., sectoral modes with high |m|) subject to geometric cancellation.

3. Circumstellar vs. Photospheric Distinction:

   • The 0.82 d⁻¹ frequency (V/R oscillation) is present only when disk material is visible.
   • The 1.644 d⁻¹ and 1.485 d⁻¹ frequencies are present in TESS photometry even during diskless phases, confirming they are photospheric in origin.
   • The 0.163 d⁻¹ signal is also present during diskless phases, confirming it is a stellar pulsation (likely a difference frequency) rather than a circumstellar orbital signal.

Significance and Claims
The authors claim that this study provides a rare, high-resolution view of a Be star disk forming from a completely diskless state, allowing for precise measurement of growth and decay timescales across multiple ionization states. The work confirms that the 0.163 d⁻¹ signal in λ Pav is a stable, long-term difference frequency arising from the interaction of two higher-frequency NRPs, rather than a direct orbital signal.

A significant finding is the decoupling of the "parent" pulsation frequencies from the spectroscopic data; while visible in photometry, they are masked in spectroscopy by fast, non-photometric LPVs and the strong difference frequency. This suggests that fast, high-order pulsations can dominate spectroscopic variability, complicating the detection of underlying NRP modes. The paper concludes that λ Pav serves as an excellent laboratory for studying non-linear mode coupling and the "hierarchical clocks" that may regulate mass ejection, noting that the 0.163 d⁻¹ signal's stability and high amplitude are unusual compared to other Be stars. The dataset provides a benchmark for theoretical models of initial mass ejection conditions and the viscous evolution of Be disks.